The objectives of this study are to determine whether there are significant changes in bone mineral density during lactation and the post-weaning periods, and whether these changes are influenced by dietary calcium intake. Two randomized calcium supplementation trials have been conducted, one during the first 6 months of lactation and the other during 6 months' post-weaning. Changes in total body, distal forearm and lumbar spine bone mineral content were determined by dual energy x-ray absorptiometry. The effects of lactation and calcium intake on calcium homeostasis and bone metabolism were also examined.